Electrically powered vehicles are often used in manufacturing and warehouse environments for transporting and manipulating articles of manufacture. Such vehicles are desirable in such environments due to their clean operation and low noise. Often such vehicles are propelled along a fixed rail or track, allowing precise control of movement along a predetermined path.
In particular, computer controlled materials transport systems are known for moving materials among various work stations of a facility. Such systems are employed, as an example, in semiconductor fabrication facilities for moving semiconductor wafers to successive work stations. In such a wafer transport system, a monorail track is routed past the work stations and a plurality of electric vehicles are mounted on the track and moveable there-along for delivering wafers to successive work stations and for removing wafers therefrom after requisite processing operations have been accomplished. The track is composed of interconnected track sections that usually include one or more routing sections or modules that are operative to provide plural paths along the track.
The vehicles on the track can operate in two modes--connected or semi-independent. In connected operation, a central controller, usually a computer, assigns destinations to vehicles and monitors operation of the whole system even when the vehicles are not at a station. The central controller monitors for collisions, obstacles and other extraordinary conditions, issuing commands to the vehicles to avoid undesired actions. While this mode allows more complex responses to conditions, it requires constant communication with the vehicles, a more powerful central controller and may have less flexible response to changing conditions.
In semi-independent mode, a central controller dispatches the vehicles and controls them when they are at a station but does not monitor the real-time operation of the system. The vehicles and/or track have facilities built in to allow the vehicles to sense their condition and respond it. This system requires some intelligence in the vehicles and may require expensive sensors to detect operational and extraordinary conditions.
Even when tracks are mounted overhead, obstacles such as hanger poles, manufacturing equipment, tools, walls and maintenance personnel can be present. The semi-independent vehicles need to sense and protect the payload from collisions with such obstacles. The avoidance of these obstacles is well known in the art.
The avoidance of other vehicles on the track has been accomplished in a number of ways; the track has been regarded as a number of zones and only one semi-independent vehicle may occupy a zone at one time, semi-automatic vehicles have been fitted with radar like capabilities and the intelligence to compute when collisions are likely, and semi-independent vehicles have treated obstacle vehicles like any other obstacle and stopped themselves. These alternatives have increased the installation cost of the system and may not allow a tailored response to other vehicles.
Curves in the track pose particularly difficult problems for semi-independent operating vehicles. Active traffic control devices have been needed at corners to assure that collisions are avoided near these features.